This invention relates to a single channel transmission and recording system for a multi-channel sensor. More particularly, it relates to a multiplexer for sampling a plurality of analog signals in a predetermined sequence and converting them to digital signals by a single delta modulator for transmission.
Various prior art devices have been used for sensing analog data and transmitting it to a remote station for analysis and storage. More particularly, such devices may have application in detecting electro-optical or infrared energy using analog sensors which are responsive to the shape of some terrain or object. The information obtained by these analog sensors is processed to obtain the maximum resolution of the object on the ground. Recent advances in aerospace vehicles and sensor technology make the consideration of reconnaissance systems with extremely high data-acquisition rates possible. It is frequently necessary to transmit this reconnaissance data to a remote location for evaluation.
Ordinarily, the analog signals are digitized for transmission and processing. The conversion of the analog signals to digital signals has a number of advantages. The digital format allows transmission of information over long distances without deterioration, since digital signals, unlike analog signals can be regenerated with only small probability of error. Since the signals are in digital form, they are not subject to phase shifting and other distortions so common with analog signals. The transmission of the signals in digital form also makes the signal to noise ratio at the receiver less critical.
Furthermore, digital information may be transmitted economically by time-division multiplexing. The frequency-division multiplexing, commonly used in transmission of analog information is thereby avoided. This avoids the complex filters which are required in frequency-division multiplexing since time-division multiplexing functions can be accomplished with digital switching circuitry. The use of digital circuitry with its relatively simple mode of switching permits fully electronic operation of the transmission systems. Thus, problems in transmission of analog information such as cross talk and mechanical contacts is avoided.
The encoding of analog information into digital information may be readily obtained by sampling the analog information at regular discrete intervals and coding the amplitude into a digital format. This procedure is commonly referred to as pulse code modulation (PCM). The art of pulse code modulation has been developed to the point where analog signals can be sampled successively and the differences transmitted. This technique is known as differential pulse-code modulation. A special form of this technique is known as delta modulation. This scheme provides for encoding of the differences in the slope of the analog signal as well as the differences in amplitude to thereby improve the coding properties of the encoders. The use of delta modulation for encoding is well described in the article by Schindler, Delta Modulation, IEEE Spectrum, Oct. 1970, p. 69.
It has been proposed to use a plurality of delta modulators each associated with a corresponding analog sensor detector for sensing infrared or electro-optical information video signals. Such a system is mounted on an in-flight data transmission system to detect an object on the ground. In such a system, the analog signal received by each of the sensors is first individually converted from analog to digital form by sampling, using a plurality of delta modulators each associated with one of the sensors. The outputs of the delta modulators are multiplexed and transmitted to a receiver station where they are decoded, demultiplexed and converted back to analog information which may be displayed and recorded by suitable means. The requirement of an individual delta modulator for each channel renders this method expensive and cumbersome.